Devices for the protection of over tensioning in spring motors for window shades and other devices have been in use for many years. One common approach to this problem is found in various forms in U.S. Pat. Nos. 350,656 to Brooks, 823,452 to Vickery, 873,438 to John, and 982,444 to Smith. Each of these patents disclose some form of a nut or other internally threaded element which is movable along a threaded shaft in response to the rotation of the shaft. A spring is fixed to the shaft and is tensioned or unwound with the rotation thereof in a given direction. The nut is positioned on the shaft so that it will engage a stop at a predetermined point when the desired number of turns of the shaft is reached. This predetermined point is made to correspond to the maximum tension which can be imposed on the spring motor without damage. Once the designated number of rotations is reached, further rotation of the shaft is prevented by engagement of the nut or other movable member with the stop.
Another approach is found in U.S. Pat. No. 1,052,689 to Pitman. In this patent, an elongated shaft is provided having a threaded portion at one end and an attachment means at the other end. A spring, acting as the motor for the window shade, is secured to the attachment means of the elongated shaft. A drum having internal threads is disposed along the threaded portion of the shaft and movable axially therealong in response to rotation of the shaft. The other end of the spring is attached to the drum, and thus, movement of the drum in one direction will cause the spring to tension and in the other direction to unwind. Means are provided to engage the drum and prevents its axial movement after a desired number of rotations of the shaft so as to provide a fixed stop to prevent the spring motor from being tensioned beyond a predetermined amount.
One limitation of each of the devices described above is that they are rather bulky in configuration and may not be readily adapted to the streamlined tubes which house modern day window shade devices. In addition, each of these over tensioning means provides a fixed stop which restricts the shaft from any further rotation in the direction in which the spring is tensioned. It is contemplated that this could present a practical problem in the use of window shades embodying such devices, particularly by those not familiar with their operation. Once the point where the stop engages is reached, the user could inadvertently pull further on the shade and create severe damage to the entire window shade assembly.
A window shade clutch assembly disclosed in a copending patent application Ser. No. 396,649 filed July 9, 1982 entitled "Window Shade Clutch Assembly" assigned to the same assignee as this invention, avoids these problems. The window shade assembly therein includes a rotatable tube, a dowel disposed longitudinally within the tube and fixed relative thereto, and a spring motor attaching to one end of the dowel. The clutch assembly consists of a shaft means connected to the other end of the spring motor and having an axial bore with internal threads which are engageable with external threads formed along at least a portion of the dowel. The shaft means includes shaft locking means engageable with retaining means formed on the interior surface of the tubed so as to permit clockwise and counterclockwise rotation of the shaft means with the tube. Since one end of the spring motor is rotating with the shaft means and tube but the other end is held from rotation by its connection to the dowel, the spring motor is tensioned by clockwise rotation of the shaft means. In addition, during such clockwise rotation, the shaft means moves axially along the dowel due to their threaded engagement with one another.
Once a predetermined number of revolutions of the shaft means is reached, corresponding to the preferred tension which may safely be applied to the spring motor, the shaft locking means is operable to disengage from the tube retaining means. The shaft means then rotates a single revolution in the clockwise direction, after which the spring motor causes it to rotate a single revolution in the counterclockwise direction so that the shaft locking means re-engages the tube retaining means. Further clockwise rotation of the tube results in alternating, single revolution clockwise-counterclockwise rotation of the shaft means thus preventing over-tensioning of the spring motor.
Although the above-described invention solves the problem of over-tensioning the window shade spring motor posed by prior art designs, it is possible that under heavy use certain elements of such window shade clutch assembly could become worn and/or weakened. It is believed that the repeated contact of the engaging portions of the shaft locking means and retaining means with one another once the spring motor is tensioned to a maximum extent could potentially create wear of such parts since the actual surface area of the engaging portions of these elements is limited. In addition, the forces transmitted to adjacent portions of the window shade by contact of the shaft locking means and retaining means could possibly result in eventual damage to those adjacent portions or even failure.
Therefore, it is an object of this invention to provide a window shade clutch assembly which prevents over tensioning of the window shade spring motor, and is designed and constructed to avoid undue wear of the cooperating parts even under relatively frequent and prolonged use.
It is another object of this invention to provide a window shade clutch assembly which prevents over tensioning of the spring motor of a window shade without creating a positive stop which restricts further rotation of the window shade tube at a predetermined point of maximum tension on the spring motor.